Hitherto, there has been proposed a fuel cell stack (see Patent Citation 1) that feeds electric power to a load device at a selected efficiency under an operating temperature, and comprises (a) a plurality of fuel cell sub-stacks and (b) an unreacted fuel conduit wherein a fuel cell in each fuel cell sub-stacks is connected to both an oxidant supply source and a fuel supply source, and the unreacted fuel conduit is connected to the fuel cells and a burner in each of the fuel cell sub-stacks, so that the unreacted fuel discharged from the fuel cells in one of the sub-stacks is permitted to flow through the unreacted fuel conduit to the burner or burners in one or two and more sub-stacks for heating the sub-stack or sub-stacks.